Goofs

During the chase scene when Shaggy, Scooby, and Velma slide down the railing of the stairway, the carpet that was on the stairs earlier during the chase is gone.

The footprints seen immediately after the wooden head were not there several moments beforehand.

When Fred and Daphne are reading the book, Daphne's eyebrows are missing until she leans in to take a closer look at what they're reading.

When the Headless Specter opens the door of the stove and Shaggy pops up from one of the holes at the top, his bottom half is missing.

Shaggy's nose is missing while he holds Scooby in his arms as Asa first stands in front of the gang.

When the Mystery Machine breaks down and Fred figures out that the radiator is the problem, the way the radiator and the rest of that side of the van makes it look like the rear end. However, the backseat of the van always has full-size doors and leaves no room for an engine which is presumed to be in the front of the vehicle.

In the scene when the gang stops in front of the mansion due to the radiator overheating, there's a bucket on a rope at the well, but in the next scene, when the gang approaches the well, the bucket is missing.

Velma tells the gang that the existence of ghosts has been scientifically proven to be a myth which is false.

In the standard side view of the Mystery Machine driving, Daphne's mouth doesn't move as she talks and Shaggy is sitting in the front when immediately after he's shown in the back with Scooby, finishing their pizza and back to playing their instruments.

Fred's mouth doesn't move when he says "Is this your ghost, guys?" His face is fixed in a smile.

Shaggy is still able to enunciate, even with a balloon in his mouth.

Fred says that the "last entry" was July 12, 1822, which appeared to be in the middle of the book, and turns the page on to that entry as he says it. He couldn't be certain that was the last entry without checking the next page.

When Velma, Shaggy and Scooby fall into the well, Velma says the water is only knee-deep. In fact, it's only ankle-deep.

Velma stands firm with Shaggy that "there's no such thing as a ghost" back at the well, but she had already seen plenty of "ghosts" before then and seemed to believe their existence at first.